INTRODUCTION 29 



air as a product of the process. This oxygen adds to the atmos- 

 pheric supply and is used by animals in respiration. The carbon 

 dioxide discharged by respiration of plants and animals is made use 

 of by plants in this synthesis of material. The excretory products 

 of animals contain nitrogen which is easily transformed into a solu- 

 ble form and absorbed by plants to be combined with the simple 

 carbohydrate, already described, to produce protein for themselves 

 and indirectly for animals. 



Zoology as Related to Man 



The values of the study of zoology may be placed in two classes : 

 cultural and practical. There is hardly a field of endeavor in the 

 realm of human activities which is not greatly influenced by zoology 

 and biology generally. The study of philosophy, the formulation 

 of our conception of religion, the comprehension of social welfare 

 problems, and many other similar intellectual and social accomplish- 

 ments are greatly facilitated by a knowledge and recognition of 

 biological principles. From the purely practical or economic side, 

 of course, agriculture, medicine, and their related sciences have 

 profited enormously. In fact, these fields are in themselves applied 

 biology. Most of the great discoveries as to the nature and control 

 of disease, the manner of inheritance of humarn characteristics, and 

 the knowledge of fundamental physiological processes occurring in 

 our own bodies have been attained by studies on other animals. 

 What is found to be true in a dog, frog, rabbit, rat, monkey, or 

 guinea pig, usually has its application to man. The loss of their 

 lives is constantly saving millions of human lives. One of the most 

 obvious uses of other animals is as a source of food supply. We 

 have only to think of such forms as mammals, birds, turtles, frogs, 

 fish, crabs, lobsters, clams, oysters, and even snails. 



Many animals are important because of their destructive tend- 

 encies in regard to articles valued by man, or to the health and life 

 of man. It is likely that the parasites which live on and in the 

 bodies of men, and on domesticated plants and animals have been 

 much more costly than the depredations of the more conspicuous 

 predators. 



Agriculture and Zoology 



A recent instance of the economic importance of zoological knowl- 

 edge is found in the saving of the entire citrus industry in Florida 



